Apparatus for powered seat track adjustment or position setting of automobile seats are extremely well known in the art. In most such track mechanisms, a unitary seat pan or seat cushion frame is mounted on the upper seat track for fore and aft movement under the control of a horizontal drive and for vertical movement of the forward and rear ends of the seat by operation of respective drives, one for each seat end. A unitary seat pan includes the seat frame and seat cushion and back frame and back cushion. Thus, any movement of the seat frame and cushion causes slight movement of both the back frame and back cushion and looseness in construction leads to a "chucking" condition prevalent in such mechanisms.
In U.S Pat. No. 2,924,265 issued Feb. 9, 1960, the seat back is separated from the seat frame to prevent vertical movement of the seat back while the seat frame and back are coupled together for horizontal motion. Such a design is commonly referred to as a "split-frame" seat track. In this apparatus a single motor is used with solenoids acuatable to clutch the selected drive to the motor to drive the seat frame and seat back in a fore and aft direction and to drive the seat frame front and rear ends in a vertical direction. No additional seat back movement is disclosed by this reference.
U.S. Pat. No. 4,432,583 issued Feb. 21, 1984 describes a seat track mechanism of generally conventional type having parallel seat tracks establishing the laterl sides of the mechanism. The mechanism provides four types of seat motion: vertical movement of the front edge of the seat, vertical movement of the rear edge of the seat, horizontal transitory movement of the seat and seat back, and pivotal motion of the seat back. These movements are all generated by lead screw mechanisms individually driven by drives such as motor drives of any generally known conventional type.
Both vertical movements of the seat frame and cushion are produced by separate lead screws by motor driven individual worm gears. Each lead screw travels within a threaded tubular member, the member being pivotally secured to one end of a link, the other end of the link being secured to a torsion bar rotated responsive to lead screw movement. A pivot arm on the torsion bar translates the rotary motion of the bar to move the seat support accordingly.
The seat track apparatus ofthe '583 patent includes a pivot connection provided between a front link and the seat frame and a sliding pivot or lost motion connection is provided between the rear link and the rear portion of the seat frame such that when the front of the seat pan is raised, the seat pan front moves in an arc toward the rear. Such movement results in the occupant of the seat having the lumbar region of his back pressed into the seat back.
Many prior art power seat tracks have an undesirable side effect in that tilting of the seat cushion causes a vertical adjustment of the rear portion of the seat cushion and thus displacement of the occupant theoretical hip point ("H" point). Each movement of the H point thus alters the postional relationship between the operator's eyes and the vehicle's mirrors, instruments and the like. Having to readjust mirrors with each actuation of the "tilt" control of the power seat is deemed undesirable
A further disadvantage of many prior art designs resides in the seat cushion having a substantially fixed longitudinal positional relationship with the seat back. Thus, shorter drivers could raise the seat cushion but could not in any way alter the horizontal position of the seat with respect to the seat back to increase occupant comfort inasmuch as typical seat cushion lengths are selected as a compromise between average sized occupants and those falling in the 95th height percentile.